Apatosaurus is not as well known as Brontosaurus, but they are one and the same animal. Brontosaurus was well known because several relatively complete skeletons of gigantic proportions had been found and displayed worldwide. However it was later found that the fossils named Brontosaurus were identical to those of Apatosaurus, which had been named earlier. Under the rules of nomenclature, an animal can have only one name-the first published one. Brontosaurus, therefore, is no more.

Apatosaurus was as long as a tennis court, though still shorter, but more heavily built, than its close relative Diplodocus. Proportionally, its neck was shorter than that of Diplodocus, and the body was not as compact, but it still had similar robust, columnlike legs. Its front legs were shorter than its back ones. As was the case with Diplodocus, its head had a long snout, and all the teeth were at the front of the mouth.

As with other sauropods, we can only puzzle at how such a small head and tiny mouth could have gathered enough food to feed an immense body. Compounding this problem, the teeth could not chew the food. They merely crushed it to extract as much energy as possible. It seems that Apatosaurus and its kin spent long periods stripping food and other food from plants, which they swallowed whole. These vast quantities of food were held in a vatlike foregut where they stewed up, breaking down to release their nutrients. At least some sauropods swallowed stones that were held in the foregut to help stir and grind the food further.

Despite its huge size, Apatosaurus was controlled by a tiny brain no bigger than a cat's. This brain made up just 0.001 percent of the animal's 25-ton (25.5 t) mass. This compares with 2.5 percent in humans.